CIE AS Chemistry (9701) exams from 2022

Revision Notes

2.3.1 Physical Properties of the Group 17 Elements

Group 17: Physical Trends

  • The group 17 elements are called halogens
  • The halogens have uses in water purification and as bleaches agents (chlorine), as flame-retardants and fire extinguishers (bromine) and as antiseptic and disinfectant agents (iodine)

Colours

  • All halogens have distinct colours which get darker going down the group

 

Group 17 - Halogen Colours, downloadable AS & A Level Chemistry revision notes

The colours of the Group 17 elements get darker going down the group

Volatility

  • Volatility refers to how easily a substance can evaporate
    • A volatile substance will have a low boiling point

 

Group 17 - Halogen Boiling Point, downloadable AS & A Level Chemistry revision notes

The boiling points of the Group 17 elements increase going down the group which indicates that the elements become less volatile

 

  • Going down the group, the boiling point of the elements increases which means that the volatility of the halogens decreases
    • This means that fluorine is the most volatile and iodine the least volatile

Group 17: Trends in Bond Strength

  • Halogens are diatomic molecules in which covalent bonds are formed by overlapping their orbitals
  • In a covalent bond, the bonding pair of electrons is attracted to the nuclei on either side and it is this attraction that holds the molecule together
  • Going down the group, the atomic size of the halogens increases
  • The bonding pair of electrons get further away from the halogen nucleus and are therefore less strongly attracted towards in

Group 17 - Covalent Bond, downloadable AS & A Level Chemistry revision notes

A covalent bond is formed by the orbital overlap of two atoms and the attraction of electrons towards the nuclei; The bigger the atom, the weaker the covalent bond

 

  • The bond strength of the halogen molecules therefore decreases going down the group

 

Group 17 - Bond Enthalpy Graph, downloadable AS & A Level Chemistry revision notes

The bond enthalpies decrease indicating that the bond strengths decrease going down the group

  • Bond enthalpy is the heat needed to break one mole of a covalent bond
  • The higher the bond enthalpy, the stronger the bond
  • An exception to this is fluorine which has a smaller bond enthalpy than chlorine and bromine
  • Fluorine is so small that when two atoms of fluorine get together their lone pairs get so close that they cause significant repulsion counteracting the attracting between the bonding pair of electrons and two nuclei

 

Group 17 - Fluorine Bond Strength, downloadable AS & A Level Chemistry revision notes

The lone pairs on fluorine get so close to each other in a fluorine molecule that they cause repulsion which decreases the bond strength

Group 17: Dipole Forces & Volatility

  • Halogens are non-metals and are diatomic molecules at room temperature
    • This means that they exist as molecules which are made up of two similar atoms, such as F2
  • The halogens are simple molecular structures with weak van der Waals’ forces between the diatomic molecules caused by instantaneous dipole-induced dipole forces

 

Group 17 - Instantaneous Dipole - Induced Dipole, downloadable AS & A Level Chemistry revision notes

The diagram shows that a sudden distribution of electrons in a nonpolar molecule can cause an instantaneous dipole. When this molecule gets close to another non-polar molecule it can induce a dipole as the cloud of electrons repel the electrons in the neighbouring molecule to the other side

  • The more electrons there are in a molecule, the greater the instantaneous dipole-induced dipole forces
  • Therefore, the larger the molecule the stronger the van der Waals’ forces between molecules
  • This is why as you go down the group, it gets more difficult to separate the molecules and the melting and boiling points increase
  • As it gets more difficult to separate the molecules, the volatility of the halogens decreases going down the group

 

Group 17 - Trend Volatility, downloadable AS & A Level Chemistry revision notes

Going down the group, the van der Waals’ forces increase due to an increased number of electrons in the molecules which means that the volatility decreases

Exam Tip

Instantaneous induced – induced dipole forces are a type of van der Waals’ forces.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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